1. Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907, USA.
2. Department of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907, USA.
3. Department of Geophysics, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401–1887, USA.
4. Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
5. Planetary Science Directorate, Southwest Research Institute, Boulder, CO 80302, USA.
6. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139–4307, USA.
7. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.
8. Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA.
9. Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, 75205 Paris Cedex 13, France.